Chapter 16: Acids and Bases

Understand the Arrhenius and Bronsted-Lowry models for acids and bases.

Understand acid strength Understand the relationship between acid strength and strength of the conjugate base Learn about the ionization of water Understand and uses the terms pH and pOH Learn methods for measuring the pH of a solution Learn to calculate the pH of solutions of strong acids.

Learn about acid-base titration.

Understand the general characteristics of buffered solutions.

Chapter 16.1: Acids and Bases

Objective: To learn about 2 models of acids and bases and the relationship of conjugate acid-base pairs.

Chapter 16: Acids and Bases

 Why do we study them?

 Lemons (citric acid)  Digest food (HCl)  Sulfuric acid is the chemical produced in the greatest amount in the U.S.

 80 billion pounds of acids are used each year To make fertilizers, detergents, plastics, pharmaceuticals, storage batteries and metals.

16-1: Properties of Acids and Bases  Were first recognized as substances that taste sour.

  Vinegar (acetic acid) Lemon (citric acid)   Bases: alkalis Bitter taste, slippery feel Most hand soaps and commercial preparations for unclogging drains are highly basic.

16-1: Arrhenius Model

 Arrhenius postulated that acids produce H+ ions in aqueous solution HCl H + + Cl strong acids  Bases produce OH- ions NaOH Na + + OH -

16-1: Bronsted-Lowry Model

  Acid is a proton donor Base is a proton acceptor HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) Acid Base conj.

Conj.

acid base Conjugate acid-base pair consists of 2 substances related to each other by the donating and accepting of a single proton.

16-1: Writing Conjugate Bases

HClO 4 H + + ClO 4 Acid conjugate base H 3 PO 4 H + + H 2 PO 4 Acid conjugate base CH 3 NH 3 + H + + CH 3 NH Acid conjugate base 2

16.2: Acid Strength

Objectives: To understand what acid strength means.

To understand the relationship between acid strength and that strength of the conjugate base.

16.2: Acid Strength

Strong acid: completely ionized or completely dissociated.

HA(aq) + H 2 O H 3 O + (aq) + A (aq) Weak acid HA(aq) + H 2 O H 3 O + (aq) + A (aq)

Figure 16.1: Representation of the behavior of acids of different strengths in aqueous solution.

Figure 16.2: Relationship of acid strength and conjugate base strength for the dissociation reaction.

Strong acids are sulfuric acid, hydrochloric acid, nitric acid, perchloric acid,

Strong Acids/weak acids

 Oxyacids: acidic hydrogen is attached to An oxygen atom.

Phosphoric acid, acetic acid, hypochlorous, Nitrous acid.

Organic acids: those with a carbon-atom Backbone, commonly contain the carboxyl group. C=O Weak acids OH

16.3: Water as an Acid and a Base  Objective: To learn about the ionization of water.

 Amphoteric: behave as either an acid or as a base.

Ionization of water: H 2 O(l) + H 2 O(l)  (aq) H 3 O + (aq) + OH -

16.3: Water as an Acid and a Base [H 3 O + ] = [OH ]= 1.0x10

-7 M At 25o C, the product of the ions is constant.

K w = [H 3 O + ] [OH ]=1.0 x 10 -14 M K w is called the ion-product constant for water.

16.3: Water as an Acid and a Base [H 3 O + ] = [OH ]= 1.0x10

-7 M A neutral solution, where [H + ] = [OH ] An acidic solution, where [H + ] > [OH ] A basic solution, where [OH ]> [H + ] K w = [H 3 O + ] [OH ]=1.0 x 10 -14 M

16.3: Water as an Acid and a Base Calculate [H+] or [OH-] as required for each of the following solutions at 25 o C.

1x10 -4 M H + 1x10 -8 M H + 1x10 -9 M OH 1x10 -3 M OH 1x10 -10 M OH 1x10 -6 M OH 1x10 -5 M H + 1x10 -11 M H +

16.4: The pH scale

  Objective: To understand pH and pOH To learn to find pOH and pH for various solutions.

 To learn to use a calculator in these calculations.

16.4: The pH scale

 Logarithm: scale that helps to represent exponents.



LOGS ARE EXPONENTS!! Using log10 ("log to the base 10"): log 10 100 = 2 is equivalent to 10 2 = 100 where 10 is the base, 2 is the logarithm (i.e., the exponent or power) and 100 is the number

10 to the x power = 1000

The pH scale provides a convenient way to represent solution acidity.

pH = -log [H+] [H+]=1x10 -3 M [OH-]=1x10 -5 M pOH= -log[OH-] pOH + pH = 14.00

16.5: Measuring pH

 Objective: To learn methods for measuring the pH of a solution.

 INDICATORS: substances that exhibit different colors in acidic and basic solutions.

Figure 16.4: Useful pH ranges for several common indicators.

16.5: Measuring pH

 Indicator paper (litmus paper): paper coated with a combination of indicators. Indicator paper turns a specific color for each pH value.

 Electronically, pH meter contains a probe that is sensitive to the [H+]. [H+] produces a voltage that appears as a reading on the pH meter.

16.7: Acid-Base Titrations

Neutralization reaction: equal amounts of H+ and OH-. Result solution with pH=7.

Titration: test that chemists perform to analyze the acid/base content of a solution.

Deliver a measured volume of a solution (w/ known concentration) the titrant into the solution being analyzed analyte .

16.7: Acid-Base Titrations

Titration: Titrant: standard solution (solution w/known concentration).

Titrant is loaded into a buret.

Figure 16.8: Microscopic picture of the solutions in the titration of 0.200

M

HNO 3 with 0.100

M

NaOH.

Buret

Figure 16.9: The pH curve for the titration of 50.0 mL of 0.200

M

HNO 3 with 0.100

M

NaOH.

Titration curve (pH Curve)

16.8: Buffered Solutions

 Objectives: To understand the general characteristics of buffered solutions.

 Buffered solution is one that resists a change in its pH even when a strong acid or base is added to it.

16.8: Buffered solutions

  Why are they important?

Many organisms can only exist in a narrow range of conditions.

 Blood (pH 7.35-7.45)

16.8: Buffered solutions

 Characteristics of a Buffer  Solution contains a weak acid HA and its conjugate base A  The buffer resists changes in pH by reacting with any added H+ or OH- so that these ions don’t accumulate  Any added H+ reacts with the base A-.

 Any added OH- reacts with the weak acid HA.

16.8: Buffered solutions

 Examples   HC 2 H 3 O 2 , NaC 2 H 3 O 2 In solution….

HCl H+ + Cl H+(aq) + C 2 H 3 O 2 HC 2 H 3 O 2 NaOH OH- + HC 2 H 3 O 2 H 2 O + HC 2 H 3 O 2

Dissociation of acid in water

K a = [H + ] [A ] [HA] K b = [NH 4 + ] [OH ] [NH 3 ] To deal w/ buffers Henderson-Hasselbalch pH=pKa + log([base]/[acid])